The present invention relates to a filter assembly, as well as a pre-filter device and a pre-filter mounting plate therefor, in particular, but not exclusively, for use in filtering intake air for gas turbines.
It is essential to correctly filter intake air to a gas turbine used for power generation purposes. In particular, because of the large volume of air drawn into the combustion system of such a gas turbine, filtration of the air is necessary to prevent fouling and/or erosion of the axial compressor blades of the turbine, as well as preventing corrosion within the low and high temperature gas path areas. For example, intake of coarse mineral dust may erode the rotor and/or stator blades of the turbine compressor, and fine sticky carbon smoke dust could create deposits on the blades. In both cases, the profile of the blade is changed from that designed for optimum operation, as a result of which turbine efficiency and power output is lost.
To avoid this problem, a gas turbine will generally have a filter section appropriate for the expected dust concentration in the area of an installation, as well as the expected climatic conditions and space limitations. A standard gas turbine filtration system can employ four or more stages of filter elements, each stage being selected to be progressively more efficient as the ambient air is drawn closer to the gas turbine intake. The final filtration stage is selected in accordance with specified gas turbine requirements. For example, one filter element per stage is normally required for every 2500 cubic feet per minute of turbine combustion intake air. This can mean that an intake filter housing can employ multiple stages of filters, with each stage holding anywhere between 12 and more than 700 elements.
A known gas turbine 1 of this type is shown in the schematic side view of FIG. 1 of the accompanying drawings. The gas turbine 1 has rotor blades 2 which are caused to rotate within a combustion part 3 of the turbine powered by gas from a fuel supply 4, so as to output electrical power at an output line (not shown). Air input to the combustion part 3 passes initially into an inlet duct 5 and then through a matrix of filter assemblies 6 in the inlet duct, so that dirt and other particulate matter contained in the air entering inlet duct 12 is prevented from reaching turbine blade 2. The cleaned air flow then passes via air transition duct 7, to the gas turbine air intake.
FIG. 2 is a diagrammatic longitudinal section, seen from the side, of a single filter assembly 6 of the gas turbine assembly of FIG. 1, shown in more detail.
The filter assembly 6 is a two-stage filter comprising an upstream, relatively coarse, pre-filter device 8 and a downstream, relatively fine, final filter device 9, which removes finer particulates that have passed through the pre-filter device 8. The air flow direction is indicated by the solid arrow in the Figure.
The final filter device 9 comprises a body part 10 that is of generally square external profile as viewed in cross-section perpendicular to the air flow axis, and one or more filter elements 24, disposed within and supported by the body part. The body part 10 has a peripheral end flange 10a at its upstream end, along the four side edges of the body part 10. The flange 10a fits closely within a substantially square-profiled mounting support or frame 11, suitably made of metal, which is turned inwardly at its downstream end to provide an end seat 11a for the peripheral flange 10a of the body part 10 of the downstream final filter device, the body part 10 extending through and beyond the downstream end opening defined by the inwardly turned seat 11a. One or more final filter retainers 12, shown very diagrammatically in FIG. 2, urge the end flange 10a of the body part 10, via a sealing gasket 14, against the seat 11a on the mounting frame 11, thereby firmly holding the final filter device in a fixed axial position relative to the mounting frame 11, extending across the opening in the downstream end of the mounting frame.
Each pre-filter device 8 is constructed as a single pre-filter media pack, comprising one or more filter elements 8a mounted within a housing box (not specifically shown in FIG. 2) that is substantially square in profile as viewed in the direction of the air flow, and is fitted within the surrounding mounting frame 11.
The upstream pre-filter device 8 is held in position, against the end flange of the downstream final filter device 9 by a clamping force provided by a plurality of retaining springs 13, for example one at each corner region of the pre-filter device 8. Each retaining spring is made from a single length of flexible wire bent to provide an end hook (not shown in FIG. 2) which passes through a hole (also not shown) formed adjacent the upstream edge of the mounting frame 11 to retain the retaining spring captive, a first link piece 13a, a coil spring section 13b, a second link piece 13c, a gripping loop 13d and another hook piece (again not shown in FIG. 2). The coil spring 13b is tensioned by pulling on the gripping loop 13d, until the second hook piece can be inserted into a hole (also not shown) in an upstream end part of the pre-filter device 8, to press the pre-filter device against the final filter device 9.
Lateral alignment between the upstream pre-filter device 8 and downstream filter device 9 is maintained by one or more alignment devices (not shown). These can be of any suitable type, one example being a peg on the mounting part of the upstream pre-filter device 8 pointing, downstream, in the direction of air flow through the filter assembly, and engaging in an aligned hole formed in the end flange 10a of the body part 10 of the downstream final filter device 9.
Since the pre-filter device 8 is required to filter out relatively coarse particles from the air flow to the gas turbine, the filtering efficiency of the pre-filter device 8 tends to reduce more rapidly than the filtering efficiency of the relatively fine final filter device 9, necessitating the more regular replacement of the upstream pre-filter device 8. This unit has an associated unit cost, which is dependent in part on the number of filter elements 8a incorporated in the upstream pre-filter unit, as well as on the mounting box within which the filter elements 8a are located, which effectively also becomes a throw-away or discardable element along with the filter element(s) 8a, since the pre-filter unit 8 is of unitary construction.
In addition, since the lateral alignment of the pre-filter device 8 with the final filter device 9 is ensured by the one or more alignment devices, such as in the form of a protruding peg on the pre-filter device 8 engaging in a correspondingly-sized hole in the end flange 10a of the downstream final filter device 9, the new replacement pre-filter devices that need to be substituted periodically for the upstream pre-filter devices 8 have to be provided with alignment pegs too. Different manufacturers use different forms of alignment device(s), even though the dimensions of the openings in the square mounting frame 11 for filter assemblies in a matrix filter are a standard size of substantially 60.5 cm by substantially 60.5 cm. This requires that the customer has to purchase the correct type of replacement pre-filter devices to suit the alignment device(s) that laterally align the used pre-filter device that is to be replaced and the final filter device to which it is fitted.
The present invention is addressed to providing a more universal arrangement which is suitable for incorporation in filter assemblies of different manufacturers or with different types of alignment device used for the lateral alignment of the pre-filter device and final filter device of a filter assembly, as well as to reducing the cost of replacement parts when the pre-filter unit has to be replaced and facilitating the fitting of replacement parts.